1. Field of the Invention
This invention relates to a pneumatic radial tire having an aspect ratio of not more than 50% for use in vehicles running on general-purpose road, circuit course and the like, and more particularly to a high-performance tire balanced and satisfying a steering stability and a resistance to hydroplaning and reducing noise.
2. Description of Related Art
For example, the pneumatic radial tire used in sport-type vehicle and having an aspect ratio of not more than 50% or so-called high-performance tire attaches much importance to steering stability and resistance to hydroplaning among performances required in usual passenger car tires, but did not put other performances such as noise reduction and the like into problem. Recently, there is a tendency of raising regulation to noise in all the countries of the world, so that a demand exists to reduce the noise even in the high-performance tire likewise the usual passenger car tire.
In order to obtain sufficient steering stability in the high-performance tire, it is known to increase a ground contacting width of a tread as compared with the usual passenger car tire. Concretely, the tread ground contacting width of the high-performance tire is generally within a range of 0.85-1.00, particularly 0.9-1.0 as a ratio to a rim width.
Although the resistance to hydroplaning is improving by the adoption of a directional pattern based on the water flowing analysis or the like, a negative ratio of tread grooves arranged in a tread (a ratio of total area of tread grooves formed in the tread portion to an area of the tread portion) is generally set to about 33-40%, which is larger than the negative ratio (about 30%) of the usual tire, from a viewpoint of making up for the degradation of the resistance to hydroplaning resulted from the size of the ground contacting width.
In addition, a groove width of a tread groove arranged in the tread portion of the high-performance tire is generally within a range of 8 mm to 10 mm or more. This is larger than a groove width (5-8 mm) of a main groove and a sub-groove in the usual tire, when the main groove extends along a circumference of the tire or obliquely at a relatively small inclination angle with respect to the circumference or the sub-groove extends along a widthwise direction of the tire or obliquely at a relatively small inclination angle with respect to the widthwise direction for ensuring a large groove area in the small groove number so as not to decrease a block rigidity of the tread.
In general, there is a relationship between the tread ground contacting width and the negative ratio considering the steering stability and the resistance to hydroplaning as mentioned below. That is, it is useful to increase the tread ground contacting width to further improve the steering stability. In this case, however, the discharging ability of water entrained in a ground contact region of the tire (particularly central region) toward the side of the tire lowers (which results from a fact that a distance from the central region to an end of the tread ground contacting width becomes long) to bring about the degradation of the resistance to hydroplaning. Therefore, it is required to increase the negative ratio in order to make up for such a degradation. On the other hand, it is useful to increase the negative ratio for more improving the resistance to hydroplaning. In the latter case, however, the substantial ground contact area of the tread decreases to degrade the steering stability. Therefore, it is required to increase the tread ground contacting width in order to make up for such degradation.
If it is intended to further improve either one of the steering stability and the resistance to hydroplaning by the conventional technique of adjusting the tread ground contacting width and the negative ratio as mentioned above, therefore, it is required to take steps for preventing the degradation of the other performance together and finally both of the tread ground contacting width and the negative ratio should be increased. For this purpose, the tire size should be made large.
When both of the tread ground contacting width and the negative ratio are increased, the total groove area of the tread grooves inclusive of the above main grooves and sub-grooves located in the ground contact region of the tread increases and also a length of a front edge portion in the ground contact region of the tire increases. This results in the occurrence of large noise because sound generated by contacting the tire with ground in the ground contact region of the tire is resonantly enlarged in a tubular region defined between the tire and the road surface by the main groove. This creates a so-called columnar resonance sound and also impact sound of a corner part of the sub-groove is increased.
It is, therefore, an object of the invention to provide pneumatic radial tires simultaneously establishing the steering stability and the resistance to hydroplaning by using a technique entirely different from the conventional technique of increasing both of the tread ground contacting width and the negative ratio and reducing the noise level.
According to the invention, there is the provision of a pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead portions, a belt superimposed about a crown portion of the carcass and a tread portion disposed on the belt and provided with a plurality of tread grooves and having an aspect ratio of not more than 50%, characterized in that when the tire is mounted onto a measuring rim defined by European Tyre and Rim Technical Organization (ETRTO) and vertically placed on a flat plate under an internal pressure of 2.5 bar at a static state and a load corresponding to 70% of a maximum load capacity determined by load index in accordance with the above internal pressure is applied thereto, a ratio of ground contacting width (PW) of tire tread to rim width (RW) is less than 0.85 and a negative ratio is not more than 25%.
The term xe2x80x9ctread groovexe2x80x9d used herein includes fine groove and sipe in addition to grooves having a relatively wide groove width such as main grooves and sub-grooves arranged in the tread portion.
In a preferable embodiment of the invention, the tread groove is a main groove extending at an inclination angle within a range of 0-45xc2x0 with respect to the circumference of the tire and/or a sub-groove extending at an inclination angle within a range of 0-44xc2x0, preferably 5-30xc2x0 with respect to the widthwise direction of the tire.
In another preferable embodiment of the invention, a groove width of a portion corresponding to not less than 80% of all main grooves arranged in the tread portion is not more than 5 mm, and a groove width of a portion corresponding to not less than 90% of all sub-grooves arranged in the tread portion is not more than 6 mm. A groove width of a portion corresponding to not less than 70% of all tread grooves including the main grooves and sub-grooves is within a range of 0.5-3.0 mm, and a maximum width of a carcass line (CW) of the carcass is smaller than the rim width (RW) and a distance (CH) between a position of a rim flange and a position of the maximum width of the carcass line shifted in a widthwise direction of the tire and projected onto an equatorial plane of the tire at section of the tire is not more than ⅓ of a distance (TH) between a position of a rim flange and a position of a sectional height of the tire projected onto the equatorial plane.